Narrator (April Hobart, CXC) Most of us have heard the story of the tortoise and the hare. The tortoise is slow and the hare is fast. But what exactly does this mean?

The speed of an object is defined as the distance it will travel in a certain amount of time. If something travels 100 feet in 10 seconds, its speed is 10 feet per second. We often talk about speeds in miles per hour in the US, or kilometers per hour in most other parts of the world.

Speeds are fun to talk about because they are easy to compare – just like the tortoise and the hare. For example, we know the fastest animal on land is the cheetah. It can reach speeds of 70 miles an hour. In the air, a Peregrine falcon is the fastest, clocking in at nearly 200 miles per hour at its top speed.

While this sounds really fast to us – and it is for humans – it's actually very slow when we compare it to, say, objects in space. For example, let's look at how fast the Earth moves around the Sun. Remember, it takes the Earth one year to make one orbit around the Sun. With this distance being 580 million miles, this means that the Earth moves on average at a speed of some 67,000 miles per hour through space.

Elsewhere in the Universe, we see things moving even faster. In the Vela pulsar, for instance, astronomers have watched as a jet of particles races away from a spinning neutron star at incredibly high speeds. They have also found evidence for giant clusters of galaxies smashing into each other at several million miles per hour.

There is, however, a limit to how fast things go. Albert Einstein himself figured out that nothing can travel faster than the speed of light. How fast is this exactly? Light travels 186,000 miles every second. Using more familiar units, this translates into over 670 million miles per hour. That is the speed limit for our Universe. Remember this when you think about what we consider fast and slow. It puts the tortoise and hare in a whole new perspective.